/*
 wiring.c - Partial implementation of the Wiring API for the ATmega8.
 Part of Arduino - http://www.arduino.cc/

 Copyright (c) 2005-2006 David A. Mellis

 This library is free software; you can redistribute it and/or
 modify it under the terms of the GNU Lesser General Public
 License as published by the Free Software Foundation; either
 version 2.1 of the License, or (at your option) any later version.

 This library is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 Lesser General Public License for more details.

 You should have received a copy of the GNU Lesser General
 Public License along with this library; if not, write to the
 Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 Boston, MA  02111-1307  USA

 $Id: wiring.c 585 2009-05-12 10:55:26Z dmellis $
 */

#include "wiring_private.h"

// the prescaler is set so that timer0 ticks every 64 clock cycles, and the
// the overflow handler is called every 256 ticks.
#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))

// the whole number of milliseconds per timer0 overflow
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)

// the fractional number of milliseconds per timer0 overflow. we shift right
// by three to fit these numbers into a byte. (for the clock speeds we care
// about - 8 and 16 MHz - this doesn't lose precision.)
#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)
#define FRACT_MAX (1000 >> 3)

volatile unsigned long timer0_overflow_count = 0;
volatile unsigned long timer0_millis = 0;
static unsigned char timer0_fract = 0;

SIGNAL(TIMER0_OVF_vect)
{
  // copy these to local variables so they can be stored in registers
  // (volatile variables must be read from memory on every access)
  unsigned long m = timer0_millis;
  unsigned char f = timer0_fract;

  m += MILLIS_INC;
  f += FRACT_INC;
  if (f >= FRACT_MAX) {
    f -= FRACT_MAX;
    m += 1;
  }

  timer0_fract = f;
  timer0_millis = m;
  timer0_overflow_count++;
}

unsigned long millis() {
  unsigned long m;
  uint8_t oldSREG = SREG;

  // disable interrupts while we read timer0_millis or we might get an
  // inconsistent value (e.g. in the middle of a write to timer0_millis)
  cli();
  m = timer0_millis;
  SREG = oldSREG;

  return m;
}

unsigned long micros() {
  unsigned long m, t;
  uint8_t oldSREG = SREG;

  cli();
  t = TCNT0;

#ifdef TIFR0
  if ((TIFR0 & _BV(TOV0)) && (t == 0))
    t = 256;
#else
  if ((TIFR & _BV(TOV0)) && (t == 0))
  t = 256;
#endif

  m = timer0_overflow_count;
  SREG = oldSREG;

  return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

void delay(unsigned long ms) {
  unsigned long start = millis();

  while (millis() - start <= ms)
    ;
}

/* Delay for the given number of microseconds.  Assumes a 8 or 16 MHz clock.
 * Disables interrupts, which will disrupt the millis() function if used
 * too frequently. */
void delayMicroseconds(unsigned int us) {
  uint8_t oldSREG;

  // calling avrlib's delay_us() function with low values (e.g. 1 or
  // 2 microseconds) gives delays longer than desired.
  //delay_us(us);

#if F_CPU >= 16000000L
  // for the 16 MHz clock on most Arduino boards

  // for a one-microsecond delay, simply return.  the overhead
  // of the function call yields a delay of approximately 1 1/8 us.
  if (--us == 0)
  return;

  // the following loop takes a quarter of a microsecond (4 cycles)
  // per iteration, so execute it four times for each microsecond of
  // delay requested.
  us <<= 2;

  // account for the time taken in the preceeding commands.
  us -= 2;
#else
  // for the 8 MHz internal clock on the ATmega168

  // for a one- or two-microsecond delay, simply return.  the overhead of
  // the function calls takes more than two microseconds.  can't just
  // subtract two, since us is unsigned; we'd overflow.
  if (--us == 0)
    return;
  if (--us == 0)
    return;

  // the following loop takes half of a microsecond (4 cycles)
  // per iteration, so execute it twice for each microsecond of
  // delay requested.
  us <<= 1;

  // partially compensate for the time taken by the preceeding commands.
  // we can't subtract any more than this or we'd overflow w/ small delays.
  us--;
#endif

  // disable interrupts, otherwise the timer 0 overflow interrupt that
  // tracks milliseconds will make us delay longer than we want.
  oldSREG = SREG;
  cli();

  // busy wait
  __asm__ __volatile__ (
      "1: sbiw %0,1" "\n\t" // 2 cycles
      "brne 1b" : "=w" (us) : "0" (us) // 2 cycles
  );

  // reenable interrupts.
  SREG = oldSREG;
}

void init() {
  // this needs to be called before setup() or some functions won't
  // work there
  sei();

  // on the ATmega168, timer 0 is also used for fast hardware pwm
  // (using phase-correct PWM would mean that timer 0 overflowed half as often
  // resulting in different millis() behavior on the ATmega8 and ATmega168)
#if !defined(__AVR_ATmega8__)
  sbi(TCCR0A, WGM01);
  sbi(TCCR0A, WGM00);
#endif
  // set timer 0 prescale factor to 64
#if defined(__AVR_ATmega8__)
  sbi(TCCR0, CS01);
  sbi(TCCR0, CS00);
#else
  sbi(TCCR0B, CS01);
  sbi(TCCR0B, CS00);
#endif
  // enable timer 0 overflow interrupt
#if defined(__AVR_ATmega8__)
  sbi(TIMSK, TOIE0);
#else
  sbi(TIMSK0, TOIE0);
#endif

  // timers 1 and 2 are used for phase-correct hardware pwm
  // this is better for motors as it ensures an even waveform
  // note, however, that fast pwm mode can achieve a frequency of up
  // 8 MHz (with a 16 MHz clock) at 50% duty cycle

  // set timer 1 prescale factor to 64
  sbi(TCCR1B, CS11);
  sbi(TCCR1B, CS10);
  // put timer 1 in 8-bit phase correct pwm mode
  sbi(TCCR1A, WGM10);

  // set timer 2 prescale factor to 64
#if defined(__AVR_ATmega8__)
  sbi(TCCR2, CS22);
#else
  sbi(TCCR2B, CS22);
#endif
  // configure timer 2 for phase correct pwm (8-bit)
#if defined(__AVR_ATmega8__)
  sbi(TCCR2, WGM20);
#else
  sbi(TCCR2A, WGM20);
#endif

#if defined(__AVR_ATmega1280__)
  // set timer 3, 4, 5 prescale factor to 64
  sbi(TCCR3B, CS31); sbi(TCCR3B, CS30);
  sbi(TCCR4B, CS41); sbi(TCCR4B, CS40);
  sbi(TCCR5B, CS51); sbi(TCCR5B, CS50);
  // put timer 3, 4, 5 in 8-bit phase correct pwm mode
  sbi(TCCR3A, WGM30);
  sbi(TCCR4A, WGM40);
  sbi(TCCR5A, WGM50);
#endif

  // set a2d prescale factor to 128
  // 16 MHz / 128 = 125 KHz, inside the desired 50-200 KHz range.
  // XXX: this will not work properly for other clock speeds, and
  // this code should use F_CPU to determine the prescale factor.
  sbi(ADCSRA, ADPS2);
  sbi(ADCSRA, ADPS1);
  sbi(ADCSRA, ADPS0);

  // enable a2d conversions
  sbi(ADCSRA, ADEN);

  // the bootloader connects pins 0 and 1 to the USART; disconnect them
  // here so they can be used as normal digital i/o; they will be
  // reconnected in Serial.begin()
#if defined(__AVR_ATmega8__)
  UCSRB = 0;
#else
  UCSR0B = 0;
#endif
}
